In order to obtain information on the molecular structure of a compound in a sample, various mass spectrometric techniques have been commonly used, such as the MS2 analysis in which an ion originating from the compound is selected as a precursor ion and subsequently dissociated by collision induced dissociation or a similar process, MS3 analysis in which a specific ion is selected as another precursor ion from the ions (product ions) obtained by the dissociation and is subjected to mass spectrometry after being dissociated, as well as MS4 or MS5 analysis in which the cycle of selecting and dissociating the precursor ion is repeated. A commonly known type of mass spectrometer capable of an MSn analysis with n being an integer equal to or greater than three is a time-of-flight mass spectrometer in which an ion trap capable of holding ions within a space, selecting the held ions according to their mass-to-charge ratios (precursor-ion selection) and performing a dissociating operation on the selected ion, is combined with a time-of-flight mass analyzer which separates ions ejected from the ion trap according to their mass-to-charge ratios and detects the separated ions.
Ion traps which employ radiofrequency electric fields can be divided into two major types: a three-dimensional quadrupole ion trap composed of an annular ring electrode and a pair of endcap electrodes arranged so that the ring electrode is sandwiched between them, and a linear ion trap having a plurality of rod electrodes (whose number is normally an even number equal to or greater than four) arranged around a central axis. Compared to the three-dimensional quadrupole ion trap, the linear ion trap has a larger space to hold ions, which means that this device is less affected by space charges due to the ions and capable of holding a larger amount of ions. Accordingly, using a linear ion trap is effective for increasing the amount of ions to be subjected to mass spectrometry and improving the sensitivity of the analysis.
Time-of-flight mass analyzers can be divided into two major types: a linear type which makes ions only fly in one direction, and a reflection type (or reflectron type) which makes ions fly a round trip by means of an ion reflector. Compared to the linear type, the reflection type is more effective for improving the mass-resolving power since it allows for a longer flight distance without significantly increasing the size of the device. Another advantage is that the time-of-flight spread which occurs due to the spread (variation) in the initial energy of the ions can be compensated for when the flying ions are made to turn around. For those reasons, particularly in recent years, reflection time-of-flight mass analyzers have been popularly used in applications which require high mass-resolving power. In the following descriptions, a reflection time-of-flight mass analyzer which uses an ion reflector is simply called a time-of-flight mass analyzer, and a time-of-flight mass spectrometer which uses such a time-of-flight mass analyzer is simply called a time-of-flight mass spectrometer (TOFMS).
Currently, the most commonly used type of ion reflector is the dual-stage reflector. In the dual-stage reflector, the reflecting field is formed by two stages of uniform electric fields which have different potential gradients along an ion beam axis. A dual-stage reflector can normally compensate for the spread in the time of flight of the ions having the same mass-to-charge ratio to the second order differential of the initial energy, whereby high mass-resolving power is obtained.
It has been known that replacing at least a portion of the uniform electric fields with a non-uniform electric field having a non-linear potential distribution is profitable in order to further compensate for the spread of the time of flight to higher-order differential components of the initial energy and thereby improve the mass-resolving power. For example, in Patent Literature 1, 2 or other documents, the present inventor and associates have proposed a novel ion reflector based on a dual-stage reflector in which a non-uniform electric field is created by slightly modifying the potential distribution within the second-stage section on the farther side in the ion reflector. This ion reflector realizes an approximately ideal potential distribution on the flight path of a packet of ions having a higher amount of initial energy than a predetermined threshold and flying in the path on the central axis of the ion reflector, whereby almost perfect isochronism is achieved (i.e. a group of ions having the same mass-to-charge ratio yet being spread in the amount of initial energy are made to simultaneously arrive at the detector, so that the time-of-flight spread is eliminated).